Future Trends of Global Agricultural Emissions of Ammonia in a Changing Climate

Because of human population growth and changes in diet, global livestock and associated ammonia $\left(N{H}_3\right)$, emissions are projected to increase through the end of the century, with possible impacts on atmospheric chemistry and climate. In this study, we propose a methodology to project global gridded livestock densities and $N{H}_3$ emissions from agriculture until 2100. Based on a downscaling method, future livestock distribution has been estimated until 2100 for three Shared Socio-economic Pathways (SSP2-4.5, SSP4-3.4, and SSP5-8.5) and used in a global process-based model (Calculation of AMmonia Emissions in ORCHIDEE, CAMEO) to estimate agricultural ammonia emissions during the 21st century. Emissions under SSP4-3.4 and SSP5-8.5 calculated by CAMEO compare well with the range estimated by the Integrated Assessment Models (IAM; 50 to 66 $TgN.y{r}^{-1}$) in the framework of the Phase 6 of the Coupled Model Intercomparison Project (CMIP6). Some opposite trends arise under SSP2.4-5 where CAMEO emissions increase consistently in response to the increasing trends in synthetic fertilizer use under this scenario. Africa is identified as the most emitting region worldwide, with $N{H}_3$ emissions ranging from 10 to 16 $TgN.y{r}^{-1}$ in 2100. Through a set of simulations, we estimated climate change as responsible for 20 % of future increase in $N{H}_3$ emissions. The produced data sets of future $N{H}_3$ emissions is an alternative option to IAM-based emissions for studies aiming at projecting the evolution of atmospheric chemistry and its impact on climate. Further model developments involving the bi-directional property of $N{H}_3$ and refinement in the future changes of agricultural practices constitute interesting perspectives.